1. Field of the Invention
This invention relates generally to a method and apparatus for producing underwater acoustic seismic signals. More particularly, it is concerned with a marine gas exploder of the type which achieves its results by generating cavitation bubbles whose collapse produces such signals.
2. Description of the Prior Art
Prior art devices of the character described typically involve the rapid separation of a pair of opposed, flat plates exposed to a body of water in a direction normal to their plane surfaces. Photographic studies reveal that when the plates are moved in opposite directions cavitation occurs adjacent both facing surfaces. The size of the two cavitation bubbles invariably differs and consequently there is a finite time separating the signals resulting from their collapse. If one of the plates is held fast, cavitation appears to occur almost exclusively at the surface of the moving plate. The Applicants have therefore surmised that existing devices of this character could be rendered more efficient by completely eliminating one of the two opposed plates and simply moving a single plate surrounded by water on all sides. Experiment verifies that this is correct. This construction also makes it possible to vary the configuration and dimensions of the plate without essentially modifying the driving mechanism. These plate characteristics significantly affect the energy content of the collapsing cavitation bubble and the acoustic signal strength which can be obtained. By contrast, in typical prior art piston and cylinder arrangements for rapidly separating opposed plates, modification of the dimensions and contour of these plates would involve substantial redesign of the entire device.
The Applicants also learned that a plate accelerated by a marine gas exploder is subjected to severe bending forces in the direction of motion, due either to sudden deceleration at the end of its forward travel or because of the non-uniform pressure distribution on its trailing surface produced by the collapsing cavitation bubble. These forces are severe enough to cause cracks or complete failure if low strength, non-resilient materials are used in plate construction.
It is therefore a general object of this invention to provide an improved method and apparatus for generating a marine seismic signal.
It is a further and more specific object of this invention to provide a method and apparatus of this character which which produces such signal through the collapse of the cavitation bubble adjacent a moving plate.
It is yet another object of this invention to provide such a method and apparatus which provide for maximum energy utilization in the formation of such a cavitation bubble.
It is a still further object of this invention to provide such a method and apparatus wherein variations in the energy content of the resultant cavitation bubble are facilitated.
Other objects and advantages of this invention will become apparent from a consideration of the detailed description to follow taken in conjunction with the drawings and as more particularly set out in the appended claims.